The Setting
To understand the cause of the crash, one needs to understand the layout of the Loop, which not only refers to Chicago's central business district, but also a rectangular pattern formed by the city's elevated trains. Some trains entering the Loop do a complete 360° circuit, and after turning around all four corners, leave on the same path they came from. Other routes enter the Loop, turning only two of the corners, and then leave on a different route. Further complicating this is the fact that some trains' routes follow a clockwise pattern around the Loop, but others go counter-clockwise.
The 1977 crash itself involved trains from two lines; one each from the Ravenswood Line and Lake–Dan Ryan Line. However, a third train, from the Evanston Express, factored in as well.
Earlier in the day of the accident, a switching issue forced dispatchers to reroute the Evanston Express to run counter-clockwise around the Loop instead of its normal clockwise route. This put it on the tracks normally used by the Ravenswood and westbound Lake–Dan Ryan trains. To avoid problems caused by this abnormal track sharing, the Ravenswood train, once entering its first turn into the Loop, was required to stop short of the first platform, waiting for the rerouted Evanston Express to clear before proceeding. Additionally, this delay meant that the Ravenswood was still in place when the Lake–Dan Ryan train arrived on these tracks, and it too was required to stop and wait for the Ravenswood to clear the track before proceeding.
Read more about this topic: 1977 Chicago Loop Derailment
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